There are two principal types of electronic circuits: analog and digital.
Analog circuits are characterized by using a range of voltages. Purely analog circuits are relatively rare these days, as most circuits incorporate at least some digital elements within them. In some cases, an analog circuit may be referred to as a linear circuit.
Digital circuitry differs from analog circuitry in that it doesn’t use a range of voltages, it uses specific voltages to perform its tasks. Most of the time, the voltage levels used in the circuit are referred to as high and low or one and zero.
While digital circuitry utilizes two voltage levels in most cases, technology is being developed that allows digital circuitry to use more than two voltage levels. Nonetheless, no matter how many voltage levels are added, those voltage levels will be discrete rather than be the range of voltages that would be used in an analog circuit.